Neurotransmitters are chemical “messengers” that function to relay electrical signals across the gap or synaptic cleft between one neuron, or nerve cell, and another. Neurotransmitters are stored in tiny sacs called vesicles, located at nerve endings. As an electrical signal arrives at a neuron's terminal, the vesicles move to the neural membrane and releases their neurotransmitter molecules into the synaptic cleft. The neurotransmitters formed in the pre-synaptic (or sending) neuron diffuse across the gap and lock onto binding sites or receptors on the membrane of a neighboring, post-synaptic (or receiving) neuron. Various biochemical processes are set into motion in the post-synaptic neuron when a neurotransmitter occupies a receptor on the surface thereof, including ion transport and release or inhibition of certain enzymes. The result is that a new electrical signal is generated in the post-synaptic neuron and the signal continues on.
Dopamine is a type of neurotransmitter that is formed in the brain and effects the processes that regulate movement, motivation, emotional response and the capacity to feel pleasure and pain. Dopamine is vital for performing balanced and controlled movements. After dopamine becomes bound to a receptor in the process of transmitting nerve signals, it is eventually released and removed from the synaptic cleft, back into the pre-synaptic neuron or glial cell by a reuptake process which operates under the influence of a protein, known as dopamine transporter (DAT), present on the neuron's outer membrane. In other words, the DAT protein acts to clear the dopamine out of the synaptic cleft, a process which is essential to normal transmission of nerve signals.
Furthermore, DAT protein is a major determinant of the intensity and duration of the dopaminergic signal. Knockout mice lacking the dopamine transporter (DAT-KO mice) display marked changes in dopamine homeostasis that result in elevated dopaminergic tone and pronounced locomotor hyperactivity (Gainetdinov et al. (2001) Proc. Natl. Acad. Sci., 98:11047-54; Hall et al. (2003) Neuropsychopharmacology, 28:620-8; Mateo et al. (2004) Proc. Natl. Acad. Sci., 101:372-7).
A number of behavorial disorders and other debilitating illnesses can be alleviated by therapeutic agents that bind to DAT proteins and inhibit dopamine reuptake. These include cocaine addiction, attention deficit disorder, depression, Parkinson's disease, obesity narcolepsy, and schizophrenia, to name a few.
Cocaine addiction continues to be a major health care concern in the United States. According to a U.S. Department of Health and Human Services report (aspe.hhs gov/health/reports/cocaine/), there are over 2 million cocaine users in the United States. In an October 2002 report, the Drug Abuse Warning Network (DAWN) indicated that there were 638,484 emergency room (ER) visits in the U.S. in 2001 related to drug abuse, among which nearly one-third were due to cocaine.
Although research has shown that cocaine binds to various neurotransmitters in the brain, including not only dopamine, but serotonin and norepinephrine, as well, the reinforcing effect of cocaine, which is a factor in the addiction, is believed to be mediated by DAT protein binding, which causes inhibition of dopamine transport. One prominent behavioral effect of cocaine and other dopamine uptake inhibitors is the stimulation of locomotor activity. There is a significant correlation among affinities for [3H] WIN 35,428 (a DAT inhibitor) binding and potencies for stimulating activity for cocaine and structurally similar compounds compared with stimulation of mouse locomotor activity (Izenwasser et al. (2004) Eur. J. Pharmacol., 263:277-83; Kunko et al. (1998) Pharmacol. Exp. Ther., 285:277-84). There is ample evidence that attenuating dopamine receptor activity with receptor agonists or antagonists will affect patient behavior in addiction (Campiani et al. (2003) J. Med. Chem., 46:3822-39; Garcia-Ladona and Fox (2003) CNS Drug Rev., 9:141-58; Schlussman et al. (2003) Pharmacol. Biochem. Behav., 75:123-31; Platt et al. (2003) Psychopharmacology (Berl)., 166:298-305; Vorel et al. (2002) J. Neurosci., 22:9595-603; Ellinwood et al. (2002) Eur. Neuropsychopharmacol., 12:407-15).
It has been reported that dopamine transporter-selective compounds may be used alone or in combination with clinically available selective serotonin reuptake inhibitors (SSRIs) for treating cocaine abuse and addiction (Owens et al. (2002) Encephale., 28:350-5; Zhang et al. (2002) J. Med. Chem., 45:1930-41; Sanchez et al. (2003) Psychopharmacology (Berl), 167:353-62; Fish et al. (2004) J. Pharmacol. Exp. Ther., 308:474-80). Indeed, Sora et al. demonstrated the importance of the concurrent involvement of the dopamine transporter and serotonin transporter (SERT) proteins in the mechanism of dependency and addiction through evidence from neurotransmitter-transporter knockout models in mice (Sora et al. (2001) Proc. Natl. Acad. Sci., 98:5300-5). In this study, it was shown, with double knockout mice models of DAT and SERT, that mice with no dopamine transporter gene and either one copy or neither copy of the serotonin transporter displayed no preference for places where they had previously received cocaine. That is, without the concurrent reuptake of dopamine and serotonin, the mice are no longer “addicted” to cocaine. It is conceivable the dopamine and serotonin transport systems may have compensated for each other, and that cocaine dependency and reward behavior may be mediated through this redundancy or compensatory mechanism.
Because cocaine interacts with a number of neurotransport processes in the brain, as noted above, the discovery of a medication that is capable of antagonizing the effect of cocaine in clinical trials, without producing sedative or other undesirable side effects has proven to be a formidable task, and to date not successfully accomplished. Indeed, a number of clinically available medications approved for other therapeutic indications, especially reuptake/transporter blockers and/or receptor agonists, have been or are being tested in clinical trials for effectiveness in curtailing cocaine craving, dependency, and addiction; however, none has yet demonstrated long term efficacy.
Imbalances in the dopaminergic system have been implicated as contributing factors in the occurrence of several neuropsychiatric disorders, including attention deficit disorder, depression and certain symptoms of schizophrenia.
Attention deficit disorder is a learning disorder involving developmentally inappropriate inattention, with or without hyperactivity. The primary signs of attention deficit disorder are a patient's inattention and impulsivity. Inappropriate inattention causes increased rates of activity or reluctance to participate or respond. A patient suffering from attention deficit disorder exhibits a consistent pattern of inattention and/or hyperactivity-impulsivity that is more frequent and severe than is typically observed in individuals at a comparable level of development. Using positron emission tomography (PET) to study the dopamine levels in the brains of young human subjects, it was found that lower levels of brain dopamine may be a contributing factor for ADHD children (Volkow et al., J. Neurosci. 21(2) RC 121, (2001). Methylphenidate (Ritalin®), a compound of similar pharmacological profile to cocaine, specifically increases the brain dopamine level, hence exhibiting the phenotypic therapeutic effects.
The mechanism by which psychostimulants act as calming agents in the treatment of attention-deficit hyperactivity disorder (ADHD) or hyperkinetic disorder is currently unknown. Experiments have shown that mice lacking the gene encoding the DAT have elevated dopaminergic tone and exhibit marked hyperactive. This activity is exacerbated by exposure to a novel environment. Additionally, these mice were impaired in spatial cognitive function, and they showed a decrease in locomotion in response to psychostimulants. This paradoxical calming effect of psychostimulants depended on serotonergic neurotransmission. The parallels between DAT knockout mice and individuals with ADHD suggest that common mechanisms may underlie some of their behaviors and responses to psychostimulants. Haloperidol has been shown to produce a sedative effect on such mice.
Depression is one of the most common of emotional disorders, having a morbidity rate of over 10% in the general population. Depression is characterized by feelings of intense sadness, despair, mental slowing, loss of concentration, pessimistic worry, agitation, and self-deprecation (Harrison's Principles of Internal Medicine, 2490-2497 (Fauci et al., eds., 14th ed. 1998)). Depression can have physical manifestations including insomnia, hypersomnia, anorexia, weight loss, overeating, decreased energy, decreased libido, and disruption of normal circadian rhythms of activity, body temperature, and endosine functions. Moreover, as many as 10% to 15% of depressed individuals display suicidal behavior. R. J. Bladessarini, Drugs and the Treatment of Psychiatric Disorders: Depression and Mania, in Goodman and Gilman's The Pharmacological Basis of Therapeutics, 431 (9th ed. 1996). Strategies to increase synaptic concentrations of dopamine have been proposed as antidepressant therapies. (See e.g., D'Aquila et al., 2000, Eur. J. Pharmacol., 405: 365-373).
Schizophrenia is considered by medical professionals to be a thought disorder, mood disorder and anxiety disorder. There is no known cure for schizophrenia. Thus, treatment is directed at the symptoms of schizophrenia and often involves administration of a combination of antipsychotic, antidepressant and antianxiety drugs. Antipsychotic drugs, such as haloperidol, have been in use for the treatment of schizophrenia since at least the 1950's. These established drugs act by blocking dopamine receptors and thereby control the hallucinations, delusions and confusion of schizophrenia. In the meantime, newer drugs have been introduced, e.g. quetiapine fumerate, and risperidone, which interact with both the dopamine and serotonin receptors, so as to treat the broad range of schizophrenia's symptoms.
One of the principal impediments to the success of treatments for schizophrenia is that patients frequently discontinue prescribed medication(s), especially those having undesirable side effects, such as blurred vision, dizziness, muscle spasms, cramps, tremors and other Parkinson-like symptoms.
The uncontrolled movements seen in sufferers of Parkinson's disease are due to the degeneration of dopamine neurons, loss of nerve terminals and consequent dopamine deficiency. Studies have shown that wild-type mice treated with a class of compounds called DAT blockers and genetic “DAT knock-out” mice are both resistant to the negative effects of specific neurotoxins, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA) on dopaminergic neurons. Therefore, a pharmacological blockade of DAT protein by potent and selective inhibitors may provide effective therapy by preventing the onset or delaying the progression of Parkinson's disease, and offer symptomatic benefits associated with increases in CNS dopamine levels.
Obesity is a disorder characterized by an abnormal increase of fat in the subcutaneous connective tissues. Among the therapeutic agents currently used to treat obesity are those that increase food intake, such as drugs that interfere with monoamine receptors, e.g., serotonin receptors, dopamine receptors, noradrenergic receptors and histamine receptors.
Narcolepsy is a neurological disorder marked by a sudden recurrent, uncontrollable compulsion to sleep, also associated with cataplexy (i.e., a sudden loss of muscle tone and paralysis of voluntary muscles associated with a strong emotion), sleep paralysis, hypnagogic hallucinations and automatic behaviors. The disease afflicts all races, females and males alike. It can vary in severity, with symptoms most commonly appearing in a person's teens and early twenties. Narcolepsy is clinically treated using central nervous system (CNS) stimulants, such as Ritalin® which exerts many of its effects through dopamine uptake blockade of central adrenergic neurons, and in particular by blocking DAT proteins.
Sydnocarb (3-(1-methyl-2-phenylethyl)-N-(phenylcarbomoyl)sydnone imine) has been discovered to have a CNS stimulatory effect, marked by an increase in locomotor activity with practically no peripheral sympathomimetic action, as described in GB Patent 1,262,830 and German Offenlegungsschrift 2028880. This discovery led to the synthesis of various sydnocarb analogues, which also act as CNS stimulants. See, for example, U.S. Pat. Nos. 4,277,609, 4,301,285, 4,371,697 and 4,446,322. However, sydnocarb, also known as mesocarb, and some of its closely related analogues are in fact derivatives of amphetamine, a highly addictive psycho-stimulant. When administered to human subjects, it is highly likely that individuals' metabolisms may convert sydnocarb back to amphetamine. Thus sydnocarb may exhibit higher abuse potential than those compounds that are without the propensity to be converted in this way. Indeed, sydnocarb is listed among the prohibited stimulants in the 2006 Guide to Prohibited Substances and Prohibited Methods of Doping, edition 6, Table 6, at 30, United States Anti-Doping Agency, Colorado Springs, Colo. (December 2005) (www.usantidoping.org).
Additionally, sydnocarb and closely related derivatives are modest reuptake inhibitors. Although having some preferential affinity towards dopamine reuptake proteins, these compounds exhibit affinity toward norepinephrine reuptake transporters, as well.
Because of the central and peripheral role played by the DAT in the dopaminergic system, it is an attractive target for therapeutic intervention against disorders and illnesses such as those described above that are alleviated by compounds that bind selectively to DAT and inhibit dopamine reuptake. Compounds that inhibit specific dopamine reuptake and lack central nervous system stimulating effects are inherently more valuable medications, as such agents would likely have fewer side effects in inducing medication dependency and drug abuse potential. Accordingly, there is an ongoing interest in the development of such compounds.